Transformation of wideband data signals from the analog-to-digital domain may require sample rates that are generally not available in pure electronic A-D converters. Using currently available technology, electronic A-D converters are limited to about 2 gigasamples/second and about 4-bits/sample of resolution. Certain applications require A-D conversion of signals using a sample rate of from 5-10 gigasamples/second, with 6-8 bits of resolution. Ideally, the sample rate of a suitable A-D converter should be from 2.5 to 4 times the maximum bandwidth of the analog signal digitized.
The fastest commercially available A-D converters are flash converters, which comprise a sample and hold circuit and a digitizer circuit. By using demultiplexing or deinterleaving techniques, the sample rate of such electronic A-D systems may be extended to about two gigasamples per second at about 6-bits of resolution.
Recognizing the limitations on bandwidth, sampling rate, and resolution of electronic A-D converters, the prior art has turned to optical devices to substantially improve upon these parameters. For example, U.S. Statutory Invention Registration H353 discloses an optical converter with expanded dynamic range, which is achieved by dividing the input signal into an optically modulated light pulse signal comprising least significant bits (LSB) and most significant bits (MSB) representations. The LSB and MSB representations are then interleaved to form a final binary representation of the input analog signal. In the apparatus used to accomplish this task, a mode-locked laser provides a source of light pulses that are conveyed to a two channel linear, electro-optic, interferometric modulator, which is fabricated on a single crystal substrate. A phase bias is introduced in one of the modulated signals. Optical detectors convert the two LSB and MSB modulated signals emerging from the modulator and an unmodulated signal into analog electrical signals. A processor determines the digital values associated with the LSB and MSB representations based on values stored in a lookup table. While this method appears to improve the resolution of the conversion process, it is apparently limited to about one to two gigasamples/second.
In U.S. Pat. No. 4,502,037, an A-D converter that includes an optical modulator is disclosed, which includes one interferometer channel for each bit of a digital output word. The output word corresponds to the magnitude of an analog input signal. The modulator applies a phase shift to each channel used to modulate light from a laser source, and the modulated light is demodulated by an array of photodetectors and comparators to produce a corresponding digital signal. The patent teaches that the response time of the A-D converter is around one nanosecond per output word, no matter what the number of stages, i.e., about one gigasamples/second.
An alternative arrangement for an optical modulator A-D converter system is disclosed in U.S. Pat. No. 4,694,276. To achieve greater resolution, light from multiple wavelength lasers is combined and transmitted through an interferometric modulator. Light modulated therein in response to an analog input signal is split into two beams of different frequency using an optical grating or prism, and the beams are directed to photodetectors. The output of the photodetectors provides two bits of resolution. The patent teaches that additional lasers of different characteristic wavelengths or additional modulators and gratings can be provided to produce greater resolution. Since each bit-pair requires either two lasers or two modulator channels and a prism/grating, this technique appears to be unduly complex and inappropriate for use in some applications.
Accordingly, it is an object of the present invention to provide an A-D converter having a sample rate of up to ten gigasamples/second and with at least 6-8 bits of resolution. A further object is to minimize the cost of the A-D converter by keeping its fabrication cost and parts count relatively low. These and other objects and advantages of the present invention will be apparent from the attached drawings and the Description of the Preferred Embodiment that follows.